A stretched film formed by stretching resin is used as an optical film serving various optical functions in various display devices utilizing the optical anisotropy thereof. For example, in a liquid crystal display device, it is known to use the stretched film as an optical compensation film for preventing the coloring of the stretched film for optical compensation such as viewing angle expansion and use the stretched film as a phase difference film doubling as a polarizing plate protection film by bonding the stretched film and a polarizer.
On the other hand, in recent years, attention has been focused on self-luminous display devices such as organic electroluminescence display devices (hereinafter, also referred to as “organic EL displays”) as new display devices. Self-luminance display devices can suppress power consumption as compared with liquid crystal display devices in which a backlight is constantly on. Further, in self-luminous display devices in which light sources corresponding to each color are respectively turned on such as organic EL displays, contrast can be further enhanced since it is not necessary to dispose color filters which cause a contrast reduction. However, since a reflector such as an aluminum plate is provided on the back side of the display in the organic EL display to enhance light extraction efficiency, there is a problem that outside light incident on the display reduces the contrast of an image by being reflected by this reflector. Thus, it is known to use a circular polarizing plate formed by bonding the stretched film and a polarizer on the front side of the display to improve brightness contrast by preventing outside light reflection. Further, such a circular polarizing plate is used also in so-called 3D liquid crystal display devices for displaying a stereoscopic image in some cases.
The above circular polarizing plate needs to be bonded in such an arrangement as to incline an in-plane slow axis of the stretched film at a desired angle with respect to an absorption axis of the polarizer.
However, a general polarizer (polarizing film) is obtained by being stretched at a high ratio in a conveying direction and the absorption axis thereof coincides with the conveying direction. A conventional phase difference film is produced by being longitudinally or laterally stretched and, in principle, an in-plane slow axis is at 0° or 90° with respect to a longitudinal direction of the film. Thus, to set an angle of inclination between the absorption axis of the polarizer and the slow axis of the stretched film at a desired angle as described above, a long polarizing film and/or a stretched film is conveyed and cut at a specific angle and film pieces have to be bonded one by one by a batch method, which has caused problems of deteriorated productivity and a reduction in the yield of products due to the adhesion of chips and the like. Particularly, in these latter days in which organic EL displays are being enlarged, if a method for obliquely conveying and cutting an obtained stretched film and bonding the cut piece to a polarizer is used, utilization efficiency of the film is deteriorated and productivity is deteriorated, wherefore improvement has been necessary.
Contrary to this, various methods for producing a long phase difference film have been proposed in which a film is stretched in an oblique direction at a desired angle and a slow axis can be freely controlled in a direction which is neither at 0° nor at 90° with respect to a width direction of the film (see, for example, patent literatures 1 to 4). In these methods, while a resin film is delivered in a direction different from a film winding direction of the resin film after stretching and conveyed with opposite end parts of the resin film gripped by pairs of gripping tools, one and the other gripping tools are moved different distances to obliquely stretch the resin film when a conveying direction is changed, thereby producing a long stretched film having a slow axis at a desired angle of exceeding 0° and below 90° with respect to a width direction of the resin film. By using such a stretched film in which the slow axis is inclined with respect to the width direction, a circular polarizing plate can be produced by bonding a long polarizing film and the stretched film (λ/4 phase difference film) in a roll-to-roll manner instead of conventional bonding by the batch method. Thus, productivity is dramatically improved and a yield can also be drastically improved.
Further, since the circular polarizing plate can be formed by bonding in a roll-to-roll manner, a utilization area of the long stretched film can be increased and production cost of the circular polarizing plate can be drastically reduced also in the case of use for large-size displays.
Further, a circular polarizing plate has been proposed which is formed by overlapping and bonding a λ/2 phase difference film and a λ/4 phase difference film such that each of in-plane slow axes thereof is at an arbitrary angle in producing a circular polarizing plate having a high degree of circular polarization (broadband circular polarizing plate) over a wider visible light wavelength region (see, for example, patent literature 5). To produce such a film in a roll-to-roll manner, the in-plane slow axis of each film needs to be inclined at an arbitrary angle with respect to a width direction. Thus, an oblique stretching device capable of stretching in a direction of an arbitrary angle by changing a stretching pattern of one oblique stretching device and producing a long stretched film having an arbitrary angle of inclination of an in-plane slow axis is more preferably used than an oblique stretching device capable of stretching only in a direction of a specific angle (see, for example, patent literature 6).
When an image in black display on an organic EL display was viewed in mounting a circular polarizing plate formed using a long stretched film produced by an oblique stretching device as described above in the organic EL display, a phenomenon of so-called “color unevenness” in which black was tinged with red or blue and the tinge differs depending on positions on the display was found.
Further, the above phenomenon was not observed in 3D liquid crystal image display devices mounted with the circular polarizing plate and found to be notably observed in organic EL displays mounted with the circular polarizing plate.
As a result of studying these problems, it was found out that, in self-luminous display devices in which light sources corresponding to each color are respectively turned on such as organic EL displays unlike liquid crystal image display devices, there were few members such as color filters which caused a contrast reduction and contrast was very high, whereas a slight variation of an optical property was notably observed as color unevenness and recognized as a problem.
As a result of further studying such problems, it was found out that, in the conventional oblique stretching device described above, there was a slight variation of an orientation angle in the case of changing a stretching angle in producing long stretched films at various stretching angles by changing the angle of the stretching direction and such a slight variation of the orientation angle was observed as color unevenness.